1. Field of the Invention
The present invention relates to an ultrasound diagnostic apparatus and an ultrasound image producing method, and in particular, to an ultrasound diagnostic apparatus and an ultrasound image producing method which synthesize a plurality of frame images having different steering angles of an ultrasonic beam to produce an update image (spatial compound image).
2. Description of the Related Art
An ultrasound diagnostic apparatus using an ultrasound image has hitherto been put into practical use in the field of medicine. In general, in this type of ultrasound diagnostic apparatus, an ultrasound image is produced by transmitting an ultrasonic beam from a transducer array having a plurality of transducers arranged therein toward a subject, receiving an ultrasonic echo from the subject by the transducer array, and electrically processing the reception signal.
In the ultrasound diagnostic apparatus, while a two-dimensional tomographic image in the subject can be acquired in real time by scanning the ultrasonic beam, a plurality of reflected waves from a fine structure in the subject interfere with one another to cause speckle noise, and speckle noise is one of the factors for causing deterioration in image quality of the ultrasound image.
As a method of reducing speckle noise, for example, a spatial compounding method disclosed in JP 2013-141519 A is known. In the spatial compounding method, a plurality of frame images are produced by transmitting an ultrasonic beam from different directions toward the same region in the subject, and a plurality of frame images are synthesized to produce one spatial compound image. Since the pattern of speckle noise fluctuates according to the transmission direction of the ultrasonic beam, a plurality of frame images produced by transmitting the ultrasonic beam from different directions are synthesized, whereby it is possible to reduce speckle noise.
For example, as shown in FIG. 12, a frame image A based on reception data acquired by transmitting and receiving an ultrasonic beam in a direction perpendicular to a transducer array, that is, in a direction of a steering angle of 0 degrees, a frame image B based on reception data acquired by transmitting and receiving an ultrasonic beam in a direction of a steering angle −a, and a frame image C based on reception data acquired by transmitting and receiving an ultrasonic beam in a direction of a steering angle +a are repeatedly produced in a three-frame cycle in the same order, and each time reception data of one frame is acquired, the frame images for three frames including previous two frame images are synthesized to produce a spatial compound image. With this, the spatial compound image made from the frame images A, B, and C corresponding to the three kinds of steering angles is constantly updated.
However, in the ultrasound image, in addition to speckle noise, patterns, such as acoustic shadow, different from an actual structure are produced, and these patterns are generated to have different shapes at different angles according to the steering angle of the ultrasonic beam to be transmitted and received. For this reason, if the inside of the subject is about to be observed by the spatial compound image to be sequentially updated while moving the transducer array along the body surface of the subject, the pattern shapes and angles of speckle noise and acoustic shadow fluctuate for each frame, and as a result, there is a problem in that artifacts in which an image vibrates like a wave occur.